In enhanced mobility MOSFET device application, relaxed Si1-xGex buffer layers have been used as virtual substrates for strained silicon layers to increase carrier mobility, K. Ismail et al., High electron mobility in modulation-doped Si/SiGe, Appl. Phys. Lett., 58, 2117, 1991; and S. Wickenhauser et al., Investigation of plastic relaxation in Si1-xGex/Si deposited by selected epitaxy, Mat. Res. Soc. Sypm. Proc. Vol. 533, 69, 1998. Si1-xGex layers relax plastically by the formation of misfit dislocations, R. Hull et al., Nucleation of misfit dislocations in strained-layer epitaxy in the GexSi1-x/Si system, J. Vac Sci. Technol., A7, 2580, 1989; Houghton, Strain relaxation kinetics in Si1-xGex/Si heterostructures, J. Appl. Phys., 70, 2136, 1991; Wickenhauser et al., Determination of the activation energy for the heterogeneous nucleation of misfit dislocations in Si1-xGex/Si deposited by selective epitaxy, Appl. Phys. Lett., 70, 324, 1997; Matthews et al., Defects in epitaxial multilayers, J. Cryst. Growth, 27, 118, 1974; and Tang et al., Investigation of dislocations in Si1-xGex/Si heterostructures grown by LPCVD, J. Cryst. Growth, 125, 301, 1992. However, during this process, threading dislocations usually are created. The existence of threading dislocations degrades device performance and reduces device yield significantly.
The current technique to produce a high quality Si1-xGex buffer layer is the growth of a several μm thick compositionally graded layer, Schaffler et al., High-electron-mobility Si/SiGe heterostructures: influence of the relaxed SiGe buffer layer, Semiconductor. Sci. Technol., 7. 260, 1992; and Fitzgerald et al., Totally relaxed GexSi1-x layers with low threading dislocation densities grown on Si substrates, Appl. Phys. Lett., 59, 811, 1991. The density of threading dislocations is still high, and the integration of a several μm thick Si1-xGex layer into commercial device fabrication is not practical. The relaxation of SiGe grown on Separation by IMplantation of Oxygen (SIMOX) wafers has also been investigated, where the Si/SiGe bilayer behaves as a free-floating foil constrained to remain flat by the substrate. However, the ratio of thicknesses between the silicon and SiGe layers must be carefully selected to move the nucleation and glide of dislocations from the SiGe layer to the silicon layer. Further, this prior art technique needs to be modified to accommodate higher germanium content in order to have utility for most technological applications, LeGouse et al., Relaxation of SiGe thin films grown on Si/SiO2 substrates, J. Appl. Phys. 75, 7240 1994.
Cavities formed in silicon and germanium and their alloys by helium implantation and annealing have been found to have a strong short-range, attractive interaction with dislocations. Introducing cavities at the SiGe/Si interface greatly enhances the relaxation rate and alters dislocation microstructures. However, reduction of threading dislocation density was not observed, Follstaedt et al., Cavity-dislocation interactions in Si-Ge and implications for heterostructure relaxation, Appl. Phys. Lett., 69, 2059, 1996. To achieve an 80% relaxation of the SiGe layer still required a one hour 1000° C. anneal.
Hydrogen implantation has been found to induce exfoliation of silicon and cause shearing of macroscopic layers of silicon, Weldon et al, On the mechanism of the hydrogen-induced exfoliation of silicon, J. Vac. Sci. Technol. B. 15, 1065, 1997. This technology has been applied to the fabrication of high-quality silicon-on-insulator (SOI) wafers, known as the SmartCut™ process. In recent publication, S. Mantl et al, Strain relaxation of epitaxial SiGe layers on Si(100) improved by hydrogen implantation, Nuclear Instruments and Methods in Physics Research B 147, 29, (1999), and H. Trinkaus et al, Strain relaxation mechanism for hydrogen-implanted Si1-xGex/Si(100) heterostructures, Appl. Phys. Lett., 76, 3552, 2000, have reported the advantages of using hydrogen implantation to increase the degree of SiGe relaxation and to reduce the density of threading dislocation. However, a relaxation of a SiGe layer of between 2000 Å and 2500 Å was reported. A SiGe layer of such thickness is not sufficient for commercial device application.
Previously, methods to form thick relaxed SiGe layer on silicon and a method to form a relaxed SiGe layer with high germanium content are disclosed in the above-cited related application, which is incorporated herein by reference. Highly relaxed SiGe films having a Ge content of 20% to 30%, or higher, by atomic ratio, with thicknesses exceeding 3000 Å have been achieved utilizing either a two layer SiGe process or a graded SiGe process.